Better Lasers and Atomic Traps Yield Better Timekeeping

Hank Hogan

Investigators from JILA, a research institute jointly administered by the National Institute of Standards and Technology (NIST) and the University of Colorado in Boulder, have developed the basics for the most precise optical atomic clock produced to date. They have devised a way to produce a “tick” 100 times more precise than those generated by current microwave-based standards.

Lead investigator Jun Ye, a JILA and NIST fellow as well as an adjoint professor of physics at the university, said that the group’s efforts should lead to improvements in timekeeping. “We expect to build the next generation of atomic clocks, [which] will be more precise and accurate than the current standards by more than a factor of 100,” he said.

In a new optical atomic clock, blue laser light cools and traps strontium atoms, which are then loaded into a lattice made of near-infrared light. There, the stationary atoms provide the basis for the ticks of what could be the most precise clock ever. The blue light and fluorescing atoms are visible in the magneto-optical trap, located inside a vacuum chamber. Photo courtesy of Tetsuya Ido and Martin M. Boyd, JILA.

The researchers used a very stable 698-nm diode laser to manipulate strontium atoms that were trapped in a lattice made of light. Developed at JILA, the laser uses a short, vertically mounted cavity that reduces vibration-induced fluctuations.

The other key component was the optical lattice, which permitted precise measurements on stationary atoms much like an ion trap does. In building the lattice, the scientists captured and cooled an atomic beam of strontium atoms, using two magneto-optical traps to chill the atoms to 1.5 μK. The trap used homemade blue and red diode lasers, and the lattice was generated by a standing wave at 813.428 nm, a value carefully selected with the characteristics of the strontium in mind, and then fine-tuned experimentally for the best results. “We measure the effect of the trap to our atomic transition quite precisely to determine the ‘magic’ wavelength,” Ye said.

As a result, tens of thousands of neutral atoms were trapped in about 100 lattice sites within a second. That number of atoms was much higher than the few absorbers that would be found in a comparable ion system, with a consequent increase in the signal strength of several orders of magnitude.

While performing spectroscopy using a custom-built instrument, the researchers achieved a linewidth of <2 Hz for an atomic transition frequency of 430 THz. That produced a resonance quality factor — the carrier frequency divided by the linewidth — of 2.4 × 10¹⁴, which the researchers reported was the highest value in any form of coherent spectroscopy. With the setup, they manipulated nuclear spins with optical fields. By applying a small magnetic field, they observed the spectroscopic transition split into 10 components.

Plans call for a doubling of the resonance quality factor within a few months. The eventual goal is an additional tenfold improvement.

Besides its role in optical atomic clocks, the technique could be useful in quantum information storage experiments. Ye noted that the number of quantum absorbers that are possible with the new method could be very attractive. “We can have nearly one million atoms all behave exactly the same way for our measurement, dramatically increasing the measurement precision.”

Science, Dec. 1, 2006, pp. 1430-1433.